1. Field of the Invention
The present invention relates generally to a vibration sensor for an automotive vehicle which can detect mechanical vibrations generated from an internal combustion engine, such as knocking, and more specifically to a vibration sensor for an automotive vehicle having a piezoelectric vibration element mounted therein in diaphragm fashion.
2. Description of the Prior Art
In general, if an engine operates with strong knocking for a long time, the knocking will have an undesired effect on the durability or other characteristics of the engine but, if the knocking is faint at relatively low engine speeds, the knocking will have a favorable effect on the characteristics of engine torque and fuel consumption rate. Heretofore, various systems have been proposed which can regulate engine spark timing in accordance with engine knocking to maintain a faint knocking at all times, improving the engine torque and fuel consumption rate. This is because engine spark timing has a close relationship to engine knocking conditions.
In such a system, a vibration sensor is indispensable for detecting enging knocking and vibration.
Conventionally, a circular sheet piezoelectric element has been used as a vibration sensor. The element is laid on a concave portion of a housing, its periphery being clamped by a flange portion of an electrode and the concave portion of the housing. When the vibration sensor of this type is attached to the engine, it vibrates in diaphragm fashion according to the vibration transmitted from the engine. The resonant frequency of the diaphragm vibrator is determined chiefly by the diameter of the diaphragm portion of the vibrator and by the clamping pressure applied to the diaphragm vibrator.
Therefore, selecting the diameter of the diaphragm portion is very critical so that the resonant frequency range is within the region of an ordinary engine knocking frequency between 5 and 9 KHz.
As a result, the assembly operation of such vibrators is time consuming and requires high dimensional precision.